The synthesis of the peptide hormone secretin and of derivatives and analogs of the peptide hormones glucagon and secretin are being investigated, and these compounds are used to study hormone-receptor interactions. For the simple binding studies of secretin, a specifically labelled radioactive derivative will be prepared. These compounds will be used to investigate the binding of the hormones to liver plasma membrane and/or fat cell "ghost" preparations which contain the hormone sensitive adenylate cyclase system related to hormone function. Hormone analogs of glucagon and secretin which contain fluorescent probes, affinity labels, etc., at selected positions on the hormones will be synthesized. These compounds will be used to study the properties of the binding site(s) and active site(s) of the hormone and the receptor. Both total synthesis and specific modification of the hormones will be investigated as approaches to the synthesis of suitable hormone analogs.